V-shaped Antennas Research Articles

Shaping the wavefront of light with high-order and short-period mode metasurfaces

We numerically investigate metasurfaces based on the well-known V-shaped antennas operating in high-order modes and short-period modes for shaping the wavefront of light. By careful arrangement of the V-shaped antennas, we design two novel super cells with large phase gradients. As a proof of concept, we design two novel metasurfaces by simulation to operate in high-order modes and short-period modes. In particular, the short-period combination can be equivalent to a metasurface operating in the second-order mode. Based on the new concept, we further arrange two azimuthal phase gradient metasurfaces to generate optical vortex beams with topological charges n = 3 and n = 2. Our results will further extend photonic research and new applications of metasurfaces.

Bending Gold Nanorods with Light.

V-shaped gold nanoantennas are the functional components of plasmonic metasurfaces, which are capable of manipulating light in unprecedented ways. Designing a metasurface requires the custom arrangement of individual antennas with controlled shape and orientation. Here, we show how highly crystalline gold nanorods in solution can be bent, one-by-one, into a V-shaped geometry and printed to the surface of a solid support through a combination of plasmonic heating and optical force. Significantly, we demonstrate that both the bending angle and the orientation of each rod-antenna can be adjusted independent from each other by tuning the laser intensity and polarization. This approach is applicable for the patterning of V-shaped plasmonic antennas on almost any substrate, which holds great potential for the fabrication of ultrathin optical components and devices.

Enhanced tunability of V-shaped plasmonic structures using ionic liquid gating and graphene

Graphene is a strong candidate for active optoelectronic devices because of its electrostatically tunable optical response. Current substrate back-gating methods are unable to sustain high fields through graphene unless a high gate voltage is applied. In order to solve this problem, ionic liquid gating is used which allows substrate front side gating, thus eliminating the major loss factors such as a dielectric layer and a thick substrate layer. On the other hand, due to its two dimensional nature, graphene interacts weakly with light and this interaction limits its efficiency in optoelectronic devices. However, V-shaped plasmonic antennas can be used to enhance the incident electric field intensity and confine the electric field near graphene thus allowing further interaction with graphene. Combining V-shaped nanoantennas with the tunable response of graphene, the operation wavelength of the devices that utilize V-shaped antennas can be tuned in situ. In the present paper, we demonstrate a graphene-based device with ionic liquid gating and V- shaped plasmonic antennas to both enhance and more effectively tune the total optical response. We are able to tune the transmission response of the device for up to 389 nm by changing the gate voltage by 3.8 V in the mid-infrared regime.

Manipulating the wavefront of light by plasmonic metasurfaces operating in high order modes

In this work, plasmonic metasurfaces with abrupt phase discontinuities operating in high order modes are investigated for manipulating the wavefront of light. We first design two types of meta-super-cells consisting of V-shaped antennas with the phase shift coverage larger than 2π. And then, we create two linear gradient phased metasurfaces using the designed cells, which exhibit exceptional abilities for light-steering functioned as meta-echelette gratings operating in high order diffraction modes, may be valuable for using in high resolution spectrographs and advantage to achieve high numerical aperture plasmonic lenses. Based on the new designed super cells we further build another two azimuthal gradient phased metasurfaces that are able to generate high order optical vortex beams. Our results could lead to wide applications in photonic research.

Mid-wave infrared metasurface microlensed focal plane array for optical crosstalk suppression.

Spatial crosstalk is one of the fundamental drawbacks of diminishing pixel size in mid-wave infrared focal plane arrays (IR-FPAs). We proposed an IR-FPA using the concept of optical phase discontinuities for substantial optical crosstalk suppression. This IR-FPA consists of asymmetrically tailored V-shaped optical antennas. Full-wave simulations confirmed major improvements in narrowing the intensity distribution of incident light beam by over 30-folds and concentrating these distributions in the central pixel of IR-FPA by achieving optical crosstalks of <1%.

Near-Field Imaging of Phased Array Metasurfaces.

Phased-antenna metasurfaces can impart abrupt, spatially dependent changes to the amplitude, phase, and polarization of light and thus mold wavefronts in a desired fashion. Here we present an experimental and computational near-field study of metasurfaces based on near-resonant V-shaped antennas and connect their near- and far-field optical responses. We show that far fields can be obtained from limited, experimentally obtained knowledge of the near fields, paving the way for experimental near-field characterization of metasurfaces and other optical nanostructures and prediction of their far fields from the near-field measurements.

Design of on-chip N-fold orbital angular momentum multicasting using V-shaped antenna array.

We design a V-shaped antenna array to realize on-chip multicasting from a single Gaussian beam to four orbital angular momentum (OAM) beams. A pattern search assisted iterative (PSI) algorithm is used to design an optimized continuous phase pattern which is further discretized to generate collinearly superimposed multiple OAM beams. Replacing the designed discrete phase pattern with corresponding V-shaped antennas, on-chip N-fold OAM multicasting is achieved. The designed on-chip 4-fold OAM multicasting exploiting V-shaped antenna array shows favorable operation performance with low crosstalk less than -15 dB.

Material effects on V-nanoantenna performance

There is great interest in aluminum based plasmonic devices due to the relatively high plasma frequency of this material as well as its low cost and self-passivating oxide layer. The passivation layer provides aluminum plasmonics with the long-term stability required for practical applications. While several studies have investigated the impact of this oxide layer on the plasmon resonances of aluminum nanostructures on glass substrates, little is known about the effect of high-refractive index substrates on these resonances. Here we present an investigation of aluminum V-shaped antennas resonant in the visible on a silicon substrate. Through comparison between numerical and experimental results, we show that the aluminium passivation layer has little effect on the antenna resonances by comparing numerical simulations both with and without. We show, however, that inclusion of the native oxide layer of the silicon substrate in numerical models is critical for achieving good agreement with experimental data. Furthermore, we computationally explore the influence of the 1.5 eV interband transition of aluminum on plasmon resonances, and find that its effect on the material properties of the resonant structures results in narrower resonances in the blue part of the spectrum than if it was not present.

Molding light propagation with arrays of V-shaped optical antennas

Molding light propagation with arrays of V-shaped optical antennas

Modeling nanoscale V-shaped antennas for the design of optical phased arrays

We present a simplified numerical method to solve for the current distribution in a V-shaped antenna excited by an electric field with arbitrary polarization. The scattered far-field amplitude, phase, and polarization of the antennas are extracted. The calculation technique presented here is an efficient method for probing the large design parameter space of such antennas, which have been proposed as basic building blocks for the design of ultrathin plasmonic metasurfaces. Our calculation is based on the integral equation method of moments and is validated by comparison to the results of finite-difference time-domain (FDTD) simulations. The computation time is approximately five orders of magnitude less than for FDTD simulations. This speed-up relies mainly on the use of the thin-wire approximation, whose domain of validity is discussed. This method can be generalized to more complex geometries such as zigzag antennas.

Ultra-thin plasmonic optical vortex plate based on phase discontinuities

A flat optical device that generates optical vortices with a variety of topological charges is demonstrated. This device spatially modulates light beams over a distance much smaller than the wavelength in the direction of propagation by means of an array of V-shaped plasmonic antennas with sub-wavelength separation. Optical vortices are shown to develop after a sub-wavelength propagation distance from the array, a feature that has major potential implications for integrated optics.

Creating V-shaped optical antennas

This paper experimentally shows that it is possible to create V-shaped optical antennas by a method based on carbon deposition controlled by a focused electron beam. The resulting structures consisted of pairs of close-spaced carbon nanorods coated with a gold layer. The electrodynamics of the V-shaped antennas have been theoretically studied by the method of finite differences in the temporal region. It is shown that the given structures can create a strong electromagnetic field in the region of the gap between the nanorods under the incidence of a light wave in the visible and IR regions. V-shaped antennas can be used as probes in near-field optical microscopy and spectroscopy.

Experimental investigation of directional characteristics for ionospheric clutter in HF surface wave radar

The unwanted radar echoes from the ionosphere are collectively called ionospheric clutter. It has proved to be the greatest impediment to achieve consistently good performance in long-range detection of surface vessels and sea-state monitoring for high-frequency surface wave radar (HFSWR). Field experimental data recorded by the HFSWR OSMAR2003 (Ocean State Monitor and Analysis Radar, manufactured in 2003) has been used in detailed investigations of the directional characteristics for this ionosphere clutter, leading to the development of effective mitigation techniques based on antenna design and adaptive signal processing. Particular attention is given to the amplitude and phase relationship among multiple spatial channels for two types of ionospheric clutter. Preliminary experimental results show that the random gain and phase variation of the antenna pattern overhead null destroyed the amplitude and phase consistency among channels. As a result, no significant measured directivity is observed in this type of specular clutter. For the incidence clutter from a lower elevation angle, it is observed that the spread clutter possesses high directivity. A nonlinear receiving array composed of multiple V-shaped antennas without deep and broad null at near-vertical incidence is proposed for adaptive ionospheric clutter suppression.

Electric field enhancing properties of the V-shaped optical resonant antennas

The electric field enhancing properties of the V-shaped optical resonant antenna are studied by using finite-difference time-domain method. Both dipolar and quadrupolar modes can be effectively excited and strong electric field enhancement in the gap of the V-shaped antenna is found. Compared with full-wave dipole antenna, the V-shaped antenna has a greater electric field enhancement, which can be attributed to the higher radiation directivity and the smaller curvature radius of the antenna arms. The more asymmetrical structure also contributes to the efficient quadrupolar excitation. The electric field enhancement of the V-shaped antenna has different dependences on the open angle of the V-shaped antenna for the dipolar and quadrupolar excitation. We obtained stronger electric field enhancing properties by using V-shaped bow-tie antennas, especially for the quadrupolar excitation. The V-shaped antenna and the bow-tie antenna can realize strongly localized and enhanced field and thus are well suitable for the use of near-field optics applications.